A reflector or a reflectarray antenna which is used in a super high frequency band is frequently applied to a large size and high gain antenna design. A reflector or a reflectarray substrate which forms a large size antenna is used as a main reflector to strongly focus an electromagnetic wave radiated from an axial feeder forwardly to obtain a high gain characteristic.
In this case, since an interval between the main reflector and the axial feeder needs to be long as much as the focal length, when an antenna is provided, a space which is occupied by the antenna is increased in order to secure the focal length. Therefore, in order to reduce the space (antenna profile) occupied by the antenna, the f/D ratio (the focal length to diameter ratio) needs to be lowered.
However, when the interval between the main reflector and the axial feeder is narrowed to lower the f/D ratio, an unnecessary electromagnetic wave which is flowed into the feeder is increased, so that reflection characteristics of the feeder undesirably deteriorate.